The growth of GaAs on (100)ScxEr1−xAs/GaAs (x=0 and 0.3) by molecular‐beam epitaxy has been studied using transmission electron microscopy. The initial stages of the three‐dimensional growth of GaAs on lattice‐matched Sc0.3Er0.7As have been investigated at different growth temperatures. Besides (100) epitactic GaAs, there are also areas of {111}‐ and {122}‐oriented GaAs observed on ScxEr1−xAs. The latter has a simple twin relationship with the neighboring (100) GaAs. Areas of {111}‐oriented GaAs have been observed only at growth temperatures above 400 °C and with moderate growth rates. The GaAs islands grown at 480 °C are faceted on {110} and {111} low‐index planes. These GaAs islands are elongated along ⟨011⟩ directions. The anisotropy of the island shape is greater at higher growth temperatures. The growth of GaAs on ScxEr1−xAs as islands is considered to be due to chemical rather than strain effects. The morphology of GaAs layers grown on ScxEr1−xAs is shown to have a strong dependence on the growth conditions. ©1995 American Institute of Physics.